Project Summary/Abstract Interpregnancy interval (IPI) is the time between birth and start of the next pregnancy (i.e., conception). A short IPI (defined as less than 18 months) is associated with risk of adverse maternal and infant health outcomes. Previous studies have found associations between short IPI and maternal anemia, premature rupture of the membranes, preterm birth, congenital anomalies and infant mortality. We propose a data analysis on all births in California between 1991-2010 to address the association between length of IPI and several maternal and infant health outcomes. Our plan consists of both a conventional analysis and a matched longitudinal analysis using women with two IPIs to serve as her own control. Our aim is to identify the most optimal and most at-risk IPIs for maternal (e.g., preterm premature rupture of membranes, gestational diabetes, preeclampsia, cesarean section) and infant outcomes (e.g., preterm birth, small for gestational age, stillbirth, infant mortality) in California. These analyses will address critical limitations from previous studies. First, the conventional analysis will allow us to examine a very large population base of more than 3.3 million deliveries by including the entire birth population of California, linked by mother with an IPI and the hospital discharge data of both the mother and infant. Second, the longitudinal matched analysis among women with more than one IPI will allow us to control for residual confounding from maternal factors. Small studies and questions about residual confounding remain the primary impediments to drawing solid inferences from the extant literature. This proposal takes advantage of unique and large datasets in a diverse state in the U.S. with information on births and co-morbidities of both mother and infant to answer a question of public health and clinical importance. The results of the California cohort will be compared to and combined with data from several other countries including Australia, Denmark, the Netherlands and the United Kingdom. The combined study population of 7.5 million births will be the largest study of its kind. The California cohort in this proposal will account for almost half the number of births ? 3.3 million. This study has the potential to inform the World Health Organization (WHO) recommendations, and provide more specific recommendations based on current demographics in these nations.